An impact sensor is used in a pedestrian protection system including an active hood system and a hood airbag in a vehicle. The impact sensor detects an impact of a collision with an object for determining whether the object is a pedestrian. When the object is determined as a pedestrian, the pedestrian protection system is activated.
An acceleration sensor arranged on the back of a bumper cover of a vehicle for detecting an impact of a collision is proposed in JP-A-2002-36994. This acceleration sensor provides a quick response because it is arranged on the back of the bumper cover. However, it is difficult to distinguish an impact due to a collision with a pedestrian from an impact due to bad road conditions.
A restive film pressure sensor is also used for detecting an impact of a collision. The pressure sensor has a structure that a conductive rubber containing carbon or metal powder is sandwiched by a pair of electrodes. The electrodes are electrically connected with each other via a resistor in a normal condition. Therefore, the sensor consumes a large amount of direct-current power and a decrease in resistance at a collision is relatively small.
Moreover, pressure switches are proposed in JP-A-09-180578 and JP-A-07-141958. In these switches, a pair of electrode lines is fixed to an inner surface of a rubber tube. A pair of conductive rubbers, which come into contact with the electrode lines, respectively. When a large amount of pressure is applied to the rubber tube, the rubber tube is flattened and the conductive rubbers are electrically connected. As a result, the pressure switch is closed. When the pressure is released, the rubber tube returns to its original form and the conductive rubbers become apart from each other. Thus, the switch becomes open.
The pressure switches are formed in a belt for providing a large sensing area. The rubber tube of the pressure switch does not elastically deform as the two electrodes come into contact with each other under a threshold pressure in principle. When the pressure applied to the rubber tube exceeds the threshold pressure, the rubber tube deforms such that the two electrodes come into close contact with each other. These pressure switches are difficult to apply for detecting an impact of a collision.